Method and apparatus for transmitting and receiving feedback information in a mobile communication system

ABSTRACT

A method and apparatus for transmitting feedback information by a User Equipment (UE) in a mobile communication system are provided. The method includes receiving information about at least one Channel State Information Reference Signal (CSI-RS); performing channel estimation based on the information about at least one CSI-RS; receiving information about at least one feedback; and transmitting at least one feedback including a channel estimation result using the information about at least one feedback, wherein the information about at least one feedback includes information about a transmission timing and a feedback mode for at least one feedback.

PRIORITY

This application claims priority under 35 U.S.C. §119(a) to a U.S.patent application filed in the United States Patent and TrademarkOffice on Oct. 12, 2011 and assigned Ser. No. 61/546,294, a U.S. patentapplication filed in the U.S. Patent and Trademark Office on Oct. 14,2011 and assigned Ser. No. 61/547,294, a Korean Patent Application filedin the Korean Intellectual Property Office on Nov. 3, 2011 and assignedSerial No. 10-2011-0113894, a Korean Patent Application filed in theKorean Intellectual Property Office on Nov. 11, 2011 and assigned SerialNo. 10-2011-0117483, a Korean Patent Application filed in the KoreanIntellectual Property Office on Jan. 30, 2012 and assigned Serial No.10-2012-0008868, and a U.S. Patent Application filed in the UnitedStates Patent and Trademark Office on Apr. 23, 2012 and assigned Ser.No. 61/636,989, the entire disclosure of each of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method and apparatus forgenerating feedback information in a cellular mobile communicationsystem including a plurality of Base Stations (BSs), and moreparticularly, to a method and apparatus for transmitting and receivingfeedback information in a Coordinated Multi-Point (CoMP) system, inwhich a plurality of BSs cooperate to support downlink transmission to aUser Equipment (UE).

2. Description of the Related Art

Mobile communication systems have been developing into high-speed,high-quality wireless packet data communication systems to provide dataservices and multimedia services beyond traditional voice-orientedservices.

Recently, various mobile communication standards, such as High SpeedDownlink Packet Access (HSDPA) and High Speed Uplink Packet Access(HSUPA) proposed by the 3^(rd) Generation Partnership Project (3GPP),High Rate Packet Data (HRPD) proposed by the 3GPP2, and 802.16 proposedby the Institute of Electrical and Electronics Engineers (IEEE), havebeen developed to support high-speed, high-quality wireless packet datatransmission services.

The 3G wireless packet data communication systems, such as HSDPA, HSUPAand HRPD, use technologies such as Adaptive Modulation and Coding (AMC)and channel-sensitive scheduling to improve transmission efficiency. InAMC and channel-sensitive scheduling, a transmitter applies a suitableModulation and Coding Scheme (MCS) at the most efficient time determinedbased on partial channel state information fed back from a receiver.

With the use of AMC, the transmitter adjusts the amount of transmissiondata according to the channel state. That is, in a poor channel state,the transmitter reduces the amount of transmission data to decrease areception error probability to a desired level. In a good channel state,the transmitter increases the amount of transmission data to increasethe reception error probability to a desired level, thereby ensuringefficient information transmission.

Additionally, with the use of channel-sensitive scheduling resourcemanagement, the transmitter selectively services a user having asuperior channel state among several users, contributing to an increasein system capacity, compared to the case in which the transmitterallocates a channel to one user and services the user. Such an increasein system capacity is called ‘multi-user diversity gain’. When AMC isused together with Multiple Input Multiple Output (MIMO), the functionof determining the number of spatial layers or the rank of atransmission signal can be adopted. A wireless packet data communicationsystem adopting AMC takes into account the number of layers for MIMOtransmission as well as a coding rate and a modulation scheme indetermining an optimum data rate.

In general, Orthogonal Frequency Division Multiple Access (OFDMA) is atechnology that can increase capacity, as compared to Code DivisionMultiple Access (CDMA). One of several reasons for increasing capacityin OFDMA is that frequency-domain scheduling is possible.

With the use of channel-sensitive scheduling, a capacity gain isobtained based on a property that a channel changes over time. Likewise,more capacity gain is achieved by utilizing another property that achannel changes in frequency. In this context, replacing CDMA used in2^(nd) Generation (2G) and 3G mobile communication systems with OFDMAfor future-generation systems has recently been studied. 3GPP and 3GPP2started to work on standardization of an evolved system using OFDMA.

FIG. 1 illustrates a cellular mobile communication system in which aTransmission (Tx)/Reception (Rx) antenna is included at the center ineach cell.

Referring to FIG. 1, in a cellular mobile communication system includinga plurality of cells, a User Equipment (UE) receives a mobilecommunication service using the aforementioned techniques from aselected cell during a semi-static time period. In FIG. 1, it is assumedthat the cellular mobile communication system includes three cells 100,110 and 120 (Cell 1, Cell 2 and Cell 3). Cell 1 provides the mobilecommunication service to UEs 101 and 102 (UE 1 and UE 2), Cell 2provides the mobile communication service to a UE 111 (UE 3), and Cell 3provides the mobile communication service to a UE 121 (UE 4). Antennas130, 131 and 132 are included at the centers of the respective cells100, 110, and 120. The antennas 130, 131, and 132 correspond to BSs orrelays.

UE 2, receiving the mobile communication service from Cell 1, isrelatively far from the antenna 130, as compared to UE 1. Moreover, Cell1 supports a relatively low data rate for UE 2 because UE 2 experiencessevere interference from the antenna 132 at the center of Cell 3.

If Cell 1, Cell 2 and Cell 3 provide mobile communication servicesindependently, they transmit Reference Signals (RSs) so that a downlinkchannel state is measured on a cell basis. In a 3GPP LTE-A system, a UEmeasures a channel state between the UE and a BS using Channel StateInformation-Reference Signals (CSI-RSs) and feeds back channel stateinformation to the BS.

FIG. 2 illustrates the positions of CSI-RSs transmitted from BSs to a UEin an LTE-A system.

Referring to FIG. 2, resources available in the LTE-A system are dividedinto equal-size Resource Blocks (RBs). The horizontal axis and verticalaxis of the resources represent time and frequency, respectively.Signals for two CSI-RS antenna ports are transmitted in the resources ofeach of RBs 200 to 219. That is, the BS transmits two CSI-RSs fordownlink measurement to the UE in the resources of the RBs 200.

In a cellular mobile communication system including a plurality of cellsas illustrated in FIG. 1, an RB at a different position is allocated toeach cell and CSI-RSs are transmitted in the resources of the allocatedRB. For example, in FIG. 1, Cell 1 transmits CSI-RSs in the resources ofthe RBs 200, Cell 2 transmits CSI-RSs in the resources of the RBs 205,and Cell 3 transmits CSI-RSs in the resources of the RBs 210. The reasonfor allocating different RBs (e.g., different time and frequencyresources) for CSI-RS transmission to different cells is to preventmutual interference between CSI-RSs from different cells.

A UE estimates a downlink channel using CSI-RSs, generates a RankIndicator (RI), a Channel Quality Indicator (CQI), and a PrecodingMatrix Index (PMI) as CSI of the estimated downlink channel, and feedsback the CSI to a BS. There are four modes defined for periodic CSIfeedback on a Physical Uplink Control Channel (PUSCH) from a UE.

1. Mode 1-0: RI, wideband CQI (wCQI)

2. Mode 1-1: RI, wCQI, wideband PMI (wPMI)

3. Mode 2-0: RI, wCQI, subband CQI (sCQI)

4. Mode 2-1: RI, wCQI, wPMI, sCQI, sPMI

The feedback timing of each piece of information in the four feedbackmodes is determined according to N_(pd), N_(OFFSET,CQI), M_(RI), andN_(OFFSET,RI) indicated by higher-layer signaling. In Mode 1-0, thetransmission period of a wCQI is N_(pd) and the feedback timing of thewCQI is determined using a subframe offset of N_(OFFSET,CQI).Additionally, the transmission period and offset of an RI areN_(pd)·_(RI) and N_(OFFSET,CQI)+N_(OFFSET,RI), respectively. Mode 1-1and Mode 1-0 have the same feedback timing, however, a PMI istransmitted together with a wCQI at the transmitting timing of the wCQIin Mode 1-1. FIG. 3 illustrates the feedback timings of an RI, a wCQI,and a PMI in Mode 1-0 and Mode 1-1. Each transmission timing isrepresented as a subframe index.

In Mode 2-0, the feedback period and offset of an sCQI are N_(pd) andN_(OFFSET,CQI), respectively. The feedback period and offset of a wCQIare H·N_(pd) and N_(OFFSET,CQI), respectively. Herein, H=J·K+1, where Kis a value indicated by higher-layer signaling and J is a valuedetermined by a system bandwidth. For instance, J is 3 for a 10-MHzsystem. Thus a wCQI is transmitted, substituting for a sCQI at every HsCQI transmissions. The feedback period and offset of an RI areM_(RI)·H·N_(pd) and N_(OFFSET,CQI)+N_(OFFSET,RI) respectively. Mode 2-1is the same as Mode 2-0 in feedback timing but different from Mode 2-0in that a PMI is transmitted together with a wCQI at the transmissiontiming of the wCQI. FIG. 4 illustrates the transmission timings of anRI, an sCQI, a wCQI, and a PMI in Mode 2-0 and Mode 2-1 under thecondition that N_(pd)=2, M_(RI)=2, J=3 (10 MHz), K=1, N_(OFFSET,CQI)=1,and N_(OFFSET,RI)=−1.

The above-described feedback timings are set for 4 or fewer CSI-RSantenna ports. For 8 CSI-RS antenna ports, two PMIs are fed back, unlikethe above cases. For 8 CSI-RS antenna ports, Mode 1-1 is further dividedinto two submodes. A first PMI is transmitted together with an RI and asecond PMI is transmitted together with a wCQI in a first submode. Thefeedback period and offset of the RI and the first PMI are defined asM_(RI)·N_(pd) and N_(OFFSET,CQI)+N_(OFFSET,RI), respectively and thefeedback period and offset of the wCQI and the second PMI are defined asN_(pd) and N_(OFFSET,CQI), respectively.

For 8 CSI-RS antenna ports, a Precoding Type Indicator (PTI) is added inMode 2-1. The PTI is transmitted together with an RI in a period ofM_(RI)·H·N_(pd) with an offset of N_(OFFSET,CQI)+N_(OFFSET,RI). If thePTI is 0, first and second PMIs and a wCQI are feedback. The wCQI andthe second PMI are transmitted at the same timing in a period of N_(pd)with an offset of N_(OFFSET,CQI). The feedback period and offset of thefirst PMI are H·N_(pd) and N_(OFFSET,CQI), respectively. H′ is indicatedby higher-layer signaling. On the other hand, if the PTI is 1, the PRIand the RI are transmitted together and the wCQI and the second PMI aretransmitted together. The sCQI is additionally fed back. The first PMIis not transmitted. The PTI and the RI have the same feedback period andoffset as those of the PTI and RI in the case in which the PTI is 0. Thefeedback period and offset of the sCQI are defined as N_(pd) andN_(OFFSET,CQI), respectively. The wCQI and the second PMI are fed backin a period of H·N_(pd) with an offset of N_(OFFSET,CQI). H is the sameas that for 4 CSI-RS antenna ports. FIGS. 5 and 6 illustratetransmission timings when PTI=0 and PTI=1 in Mode 2-1 for 8 CSI-RSantenna ports under the condition that N_(pd)=2, M_(RI)=2, J=3 (10 MHz),K=1, H′=3, N_(OFFSET,CQI)=1, and N_(OFFSET,RI)=−1.

The conventional CSI feedback technology is based on the premise that aUE transmits a single CSI feedback, without regard to a multi-CSIfeedback situation for CoMP transmission, that is, simultaneoustransmissions from a plurality of transmission points.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made to solve theabove-stated problems occurring in the prior art, and an aspect of thepresent invention provides a method and apparatus for transmitting andreceiving feedback information in a mobile communication system.

Another aspect of the embodiments of the present invention is to providea multi-Channel State Information (CSI) feedback method and apparatusfor Coordinated Multi-Point (CoMP) transmission in a mobilecommunication system.

In accordance with an embodiment of the present invention, a method fortransmitting feedback information by a User Equipment (UE) in a mobilecommunication system is provided. The method includes receivinginformation about at least one Channel State Information ReferenceSignal (CSI-RS); performing channel estimation based on the informationabout at least one CSI-RS; receiving information about at least onefeedback; and transmitting a feedback including a channel estimationresult using the information about at least one feedback, wherein theinformation about at least one feedback includes information about atransmission timing and a feedback mode for the feedback.

In accordance with another embodiment of the present invention, a methodfor receiving feedback information by a central control device managingat least one cell in a mobile communication system is provided. Themethod includes transmitting information about at least one ChannelState Information Reference Signal (CSI-RS) to a User Equipment (UE);transmitting information about at least one feedback to the UE; andreceiving at least one feedback transmitted based on the informationabout at least one feedback from the UE, wherein the information aboutat least one feedback includes a channel estimation result generatedbased on the information about at least one CSI-RS and includesinformation about a transmission timing and a feedback mode for the atleast one feedback.

In accordance with another embodiment of the present invention, a UserEquipment (UE) in a mobile communication system is provided. The UEincludes a transmitter; a receiver for receiving information about atleast one Channel State Information Reference Signal (CSI-RS) andinformation about at least one feedback; and a controller for performingchannel estimation based on the information about at least one CSI-RSand, upon receipt of the information about at least one feedback,controlling the transmitter to transmit at least one feedback includinga channel estimation result using the information about at least onefeedback, wherein the information about at least one feedback includesinformation about a transmission timing and a feedback mode for the atleast one feedback.

In accordance with a further embodiment of the present invention, acentral control device managing at least one cell in a mobilecommunication system is provided. The central control device includes atransmitter for transmitting information about at least one ChannelState Information Reference Signal (CSI-RS) and information about atleast one feedback to a User Equipment (UE); and a receiver forreceiving at least one feedback transmitted based on the informationabout at least one feedback from the UE, wherein the information aboutat least one feedback includes a channel estimation result generatedbased on the information about at least one CSI-RS and includes atransmission timing and a feedback mode for the at least one feedback.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, objects, features and advantages of certainembodiments of the present invention will be more apparent from thefollowing detailed description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a cellular mobile communication system in which aTransmission (Tx)/Reception (Rx) antenna is included at the center ineach cell;

FIG. 2 illustrates the positions of Channel State Information-ReferenceSignals (CSI-RSs) that Base Stations (BSs) transmit to a User Equipment(UE) in a typical Long Term Evolution-Advanced (LTE-A) system;

FIG. 3 illustrates feedback timings of a UE in Mode 1-0 or Mode 1-1 inthe typical LTE-A system;

FIG. 4 illustrates feedback timings of a UE in Mode 2-0 or Mode 2-1 inthe typical LTE-A system;

FIG. 5 illustrates feedback timings of a UE in Mode 2-1 for 8 CSI-RSantenna ports when a Precoding Type Indicator (PTI) is 0 in the typicalLTE-A system;

FIG. 6 illustrates feedback timings of a UE in Mode 2-1 for 8 CSI-RSantenna ports when the PTI is 1 in the typical LTE-A system;

FIG. 7 illustrates the configuration of a cellular mobile communicationsystem according to an embodiment of the present invention;

FIG. 8 illustrates the positions of CSI-RSs that BSs transmit to a UEaccording to an embodiment of the present invention;

FIG. 9 illustrates feedback timings of a UE according to anotherembodiment of the present invention;

FIG. 10 illustrates feedback timings of a UE according to a thirdembodiment of the present invention;

FIG. 11 illustrates feedback timings of a UE according to the thirdembodiment of the present invention;

FIG. 12 illustrates feedback timings of a UE according to the thirdembodiment of the present invention;

FIG. 13 illustrates feedback timings of a UE according to a fourthembodiment of the present invention;

FIG. 14 illustrates feedback timings of a UE according to the fourthembodiment of the present invention;

FIG. 15 is a block diagram of a UE according to an embodiment of thepresent invention;

FIG. 16 is a flowchart illustrating an operation of the UE according toan embodiment of the present invention;

FIG. 17 is a block diagram of a central control device according to anembodiment of the present invention; and

FIG. 18 is a flowchart illustrating an operation of the central controldevice according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

While embodiments of the present invention will be described in detailin the context of an Orthogonal Frequency Division Multiplexing (OFDM)wireless communication system, especially conforming to the 3^(rd)Generation Partnership Project (3GPP) Evolved UMTS Terrestrial RadioAccess (EUTRA) standard, it will be understood to those skilled in theart that the subject matter of the present invention is applicable toother communication systems having a similar technical background andchannel configuration with a slight modification made without departingmuch from the scope and spirit of the present invention.

A cellular mobile communication system includes a plurality of cells ina limited area. Each cell is provided with Base Station (BS) equipmentcontrolling mobile communication within the cell. The BS equipmentprovides a mobile communication service to User Equipments (UEs) withinthe cell. A specific UE receives the mobile communication service onlyfrom one semi-statically determined cell. This system is called anon-Coordinated Multi-Point (CoMP) system.

In the non-CoMP system, the data rates of UEs within a cell vary basedon their locations in the cell. A UE at a cell center generally has ahigh data rate, whereas a UE at a cell edge generally has a low datarate.

The opposite concept of the non-CoMP system is a CoMP system. In a CoMPsystem, a plurality of cells coordinate data transmission to a UElocated at a cell edge. Thus, a better-quality mobile communicationservice is provided to the UE, as compared to the non-CoMP system. Anembodiment of the present invention provides a method and apparatus fortransmitting feedback information in a CoMP system, taking into accountDynamic cell Selection (DS), Dynamic cell Selection with DynamicBlanking (DS/DB), and Joint Transmission (JT). In DS, a UE measures achannel state on a cell basis and transmits feedback information relatedto the measured channel states to a BS. The BS then dynamically selectsa cell that will transmit downlink data to the UE. In DS/DB, a specificcell discontinues data transmission, to mitigate interference withanother cell. JT is a technique of simultaneously transmitting data froma plurality of cells to a specific UE. That is, to overcome theproblems, an embodiment of the present invention designs a feedbackstructure so as to efficiently apply DS, DS/DB, or JT to the LTE-Asystem.

FIG. 7 illustrates the configuration of a cellular mobile communicationsystem according to an embodiment of the present invention. In FIG. 7,it is assumed that the cellular mobile communication system includesthree cells 300, 310 and 320 (Cell 1, Cell 2 and Cell 3). In anembodiment of the present invention, a cell refers to a datatransmission area that a specific transmission point can service. Eachtransmission point is a Remote Radio Head (RRH) sharing a cellIdentifier (ID) with a macro BS within a macro area or a macrocell orpicocell having a different cell ID.

A central control device is a device that can transmit and receive datato and from a UE and process the transmission and received data. If eachtransmission point is an RRH sharing a cell ID with a macro BS, themacro BS is called a central control device. However, if eachtransmission point is a macrocell or a picocell having a different cellID, a device that manages cells integrally is called a central controldevice.

Referring to FIG. 7, in the cellular mobile communication system, first,third and fourth UEs 301, 311 and 321 (UE 1, UE 3 and UE 4) receive datafrom their nearest cells among Cell 1, Cell 2 and Cell 3 and 320 and asecond UE 302 (UE 2) receives data from Cell 1, Cell 2 and Cell 3 byCoMP. UE 1, UE 3 and UE 4, which receive data from their nearest cells,estimate channels using Channel State Information Reference Signals(CSI-RSs) received from the cells and transmit related feedbackinformation to a central control device 330. However, UE 2 estimateschannels received from all of Cell 1, Cell 2 and Cell 3 because itreceives data from Cell 1, Cell 2 and Cell 3 by CoMP. Accordingly, thecentral control device 330 allocates resources for three CSI-RSscorresponding to the three cells 300, 310 and 320 to UE 2, for channelestimation of UE 2. A method for allocating CSI-RSs to UE 2 by thecentral control device 330 will be described below with reference toFIG. 8.

FIG. 8 illustrates the positions of CSI-RSs that BSs transmit to a UEaccording to an embodiment of the present invention.

Referring to FIG. 8, the central control device 330 allocates resources401, 402 and 403 to three CSI-RSs and transmits CSI-RSs in the allocatedresources so that UE 2 for which CoMP transmission is supported canestimate channels from the three cells 300, 310 and 320 and channelscarrying control information and system information. That is, referencenumeral 401 denotes resources allocated to a CSI-RS used for channelestimation of Cell 1, reference numeral 402 denotes resources allocatedto a CSI-RS used for channel estimation of Cell 2, and reference numeral403 denotes resources allocated to a CSI-RS used for channel estimationof Cell 3. A set of resources allocated to at least one CSI-RStransmitted for channel estimation of a CoMP UE or a set of cellscorresponding to the CSI-RS resources is called a measurement set.

Embodiment 1

If a measurement set including a plurality of cells is allocated to aUE, CSI feedback regarding the measurement set can be considered largelyin two methods. One is to feedback CSI in a different CSI feedback modeat a different timing for each cell of the measurement set. For example,if the measurement set is {Cell-1, Cell-2, Cell-3}, a feedback mode andtiming are allocated to each cell in the per-cell CSI feedback method.That is, an allocation method described in Example 1 is used:

Example 1

-   -   Cell-1: (Mode 1-1, N_(pd)=10, M_(RI)=2, N_(OFFSET,CQI)=0,        N_(OFFSET,RI)=−1)    -   Cell-2: (Mode 1-1, N_(pd)=10, M_(RI)=2, N_(OFFSET,CQI)=2,        N_(OFFSET,RI)=−1)    -   Cell-3: (Mode 1-1, N_(pd)=10, M_(RI)=2, N_(OFFSET,CQI)=4,        N_(OFFSET,RI)=−1)

When feedback modes and timings are allocated to a CoMP UE on a cellbasis as illustrated in Example 1, it is necessary to consider collisionbetween feedback timings. In Example 1, collision can be avoided throughan appropriate combination of a transmission period and an offset.However, a wrong setting of a transmission period and an offset, acarrier aggregation situation, or a Time Division Duplexing (TDD)situation can cause collision between feedback timings. According to anembodiment of the present invention, a feedback is first transmitted toa cell having the highest priority in a collision. That is, informationabout a cell having a higher priority is fed back first, in case one ofan RI, a PMI, and a CQI collides with another. This method isadvantageous in that a feedback can be transmitted without consideringCoMP even in the case of feedback collision between cells and thus a UEcan receive data from at least one cell without performance degradationcaused by multi-feedback collision.

Cells are prioritized by signaling a measurement set and the prioritylevels of cells included in the measurement set to a UE by a BS andprioritizing the cells based on the priority levels of the cells by theUE. Cells can also be prioritized according to CSI-RS resource indexesfor cells included in a measurement set or a higher priority level canbe assigned to a cell with a shorter feedback period.

In FIG. 8, the central control device 330 allocates additional resourcesfor interference measurement to UE 2. The amount of data that UE 2 canreceive per unit time is affected by the magnitude of interference aswell as the strength of a signal. Accordingly, the central controldevice 330 allocates an Interference Measurement Resource (IMR)configured only for interference measurement to UE 2 so that UE 2 canaccurately measure interference.

The BS allocates one IMR to a UE so that the UE measures the magnitudeof common interference involved in the signal components of all CSI-RSsof a measurement set. The BS can also allocate a plurality of IMRs to aUE so that the UE measures interference in various situations.

Referring to FIG. 8, UE 2 measures signals received from the three cells300, 310, and 320 using the allocated three CSI-RS resources 401, 402and 403, and measures interference involved in the signals received fromthe three cells 300, 310 and 320 using an allocated IMR 410. The BScontrols signal transmissions from neighboring cells using the IMR 410so that much of interference with UE 2 is reflected in the IMR 410.

In the case in which a measurement set including a plurality of cellsand one or more IMRs are allocated to a UE, an embodiment of the presentinvention considers the types of feedbacks to be transmitted to a BS, amethod for generating and transmitting feedback information, and a UEfeedback operation for the case in which different types of feedbackscollide at a specific timing.

When a measurement set including a plurality of cells and one or moreIMRs are allocated to a UE, a BS allocates a plurality of feedbacks tothe UE regarding signals and interference that can be generated. The UEthen generates feedback information according to the allocated feedbacksand transmits the feedback information to the BS at a predeterminedfeedback transmission timing.

For example, if the measurement set allocated to the UE is {CSI-RS-1,CSI-RS-2}, CSI-RS-1 and CSI-RS-2 are CSI-RSs transmitted by Cell-1 andCell-2, respectively, the BS allocates one IMR to the UE, and theallocated IMR reflects interference from cells other than the cells ofthe measurement set, the BS and the UE operates as follows.

The BS allocates FeedBacks (FBs) regarding four signals and interferenceas illustrated in Table 1 below and the UE generates and transmitsfeedback information according to the allocated FBs.

TABLE 1 Signal component Interference Considerations FB 1 Cell-1 IMR +Cell-2 No blanking FB 2 Cell-1 IMR Blanking of Cell-2 FB 3 Cell-2 IMR +Cell-1 No blanking FB 4 Cell-2 IMR Blanking of Cell-1

In Table 1, IMR+Cell-2 means that the UE considers the sum ofinterference measured in the IMR and interference measured in CSI-RS-2received from Cell 2 as a feedback for FB 1. That is, FB 1 indicates aCSI feedback for the case in which a signal is received from Cell-1, andCell-2 and cells reflected in the IMR other than the cells of themeasurement set cause interference. FB 2 indicates a CSI feedback forthe case in which a signal is received from Cell-1 and only cells otherthan the cells of the measurement set cause interference because Cell-2is in blanking state and thus does not transmit a signal. The CSI of FB1 and FB 2 include individual RIs, PMIs and CQIs or a common RI, acommon PMI, and individual CQIs.

Similarly, FB 3 and FB 4 commonly indicate a CSI feedback for the casein which a signal is received from Cell-2, and FB 3 and FB 4 are forblanking of Cell-1 and non-blanking of Cell-1, respectively. FB 3 and FB4 have individual RIs, individual PMIs, and individual CQIs or a commonRI, a common PMI, and individual CQIs. That is, FBs are designed to havea common RI and a common PMI for the same signal component and separateCQIs for different interference situations.

When the UE generates feedback information based on various signals andinterference as illustrated in Table 1 and transmits the feedbackinformation to the BS, Timing 1 to Timing 4 are assigned as thetransmission timings of the feedback information as illustrated in Table2 below. The feedback information is prioritized in order to preventcollision between the transmission timings of different feedbackinformation. As feedback information is prioritized, feedbackinformation with a higher priority is transmitted despite the collisionbetween the transmission timings of feedback information, while feedbackinformation with a lower priority is be transmitted.

TABLE 2 Signal Feedback Feedback component Interference timing priorityFB 1 Cell-1 IMR + Cell-2 Timing 1 1 FB 2 Cell-1 IMR Timing 2 3 FB 3Cell-2 IMR + Cell-1 Timing 3 2 FB 4 Cell-2 IMR Timing 4 4

To prevent collision between the transmission timings of a plurality ofpieces of feedback information, the feedback information is prioritizedin various methods. One method of prioritizing feedbacks is for a BS toprioritize feedback information according to the priority index of thefeedback information.

For example, when the priority indexes illustrated in the last column ofTable 2 are assigned to a plurality of FBs and a collision occursbetween the transmission timings of FB 1 with priority index 1 and FB 2with priority index 3, the UE transmits a CSI feedback of FB 1 withouttransmitting a CSI feedback of FB 2.

The BS transmits feedback priority indexes to the UE by RRC signaling asillustrated in the last column of Table 2 or FBs are prioritized in theorder of FB allocation numbers without assigning additional priorityindexes. That is, if FB 1 collides with FB 2, a feedback for FB 1 istransmitted with priority at a given timing, while a feedback for FB 2is not transmitted. To generalize this operation, for indexes m and n(where m>n), a feedback is transmitted always for FB m, while a feedbackis not transmitted for FB n, at a colliding transmission timing.

Another method of prioritizing FBs is that feedback priorities aredetermined based on CSI-RS resource indexes corresponding to signalcomponents for which feedback information is configured. That is, when acollision occurs between the transmission timings of two pieces offeedback information, the UE transmits feedback information with a lowerCSI-RS resource index, while not transmitting feedback information witha higher CSI-RS resource index. For example, in Table 2, if the CSI-RSresource indexes for Cell-1 and Cell-2 are 1 and 2, respectively,feedback information for Cell-1 is transmitted with priority overfeedback information for Cell-2.

A third method for prioritizing feedback information is to determine thepriority level of feedback information according to the type ofinterference involved in the feedback information. Feedback informationfor a case in which interference is measured only in an IMR has priorityover feedback information for a case in which interference is measuredin a plurality of resources such as “IMR+Cell-1”, as illustrated inTable 2. In the opposite case, a case in which interference is measuredin more resources has a higher feedback priority level than a case whereinterference is measured in fewer resources. Or the BS assigns priorityindexes according to interference types and thus feedbacks areprioritized based on the assigned priority indexes. For example, whenthe BS assigns index 1 to a case where the UE measures interference onlyin an IMR and index 1 to a case where the UE considers both interferencein the IMR and interference with Cell-1 in Table 2, the UE prioritizesfeedbacks according to the interference indexes.

Table 3 below illustrates a situation in which a set of two IMRs {IMR 1,IMR 2} and a plurality of related FBs are assigned to a UE. IMR 1 andIMR 2 represent interference measurement resources reflecting differentinterference situations. Another method of prioritizing feedbacksaccording to interference types is to give a higher priority level tofeedback information with a lower IMR index than feedback informationwith a higher IMR index, when a BS assigns a plurality of IMRs to a UEas illustrated in Table 3. For example, feedback information of FB 1with a lower IMR index (e.g., IMR 1) is transmitted, while feedbackinformation of FB 2 with a higher IMR index (e.g., IMR 2) is nottransmitted, at a transmission timing at which FB 1 collides with FB 2in Table 3.

TABLE 3 Signal Feedback component Interference timing FB 1 Cell-1 IMR 1Timing 1 FB 2 Cell-1 IMR 2 Timing 2 FB 3 Cell-2 IMR 1 Timing 3 FB 4Cell-2 IMR 2 Timing 4

A final method of prioritizing feedbacks uses the afore-described twofeedback prioritizing methods (i.e. the CSI-RS resource index-basedfeedback prioritizing method and the method of prioritizing feedbacksaccording to interference types) in combination. When a collision occursbetween the transmission timings of two feedbacks, feedback informationwith a lower CSI-RS resource index is transmitted with priority.However, if the two feedbacks have the same CSI-RS resource index, theyare prioritized based on interference types.

However, if a collision occurs between the transmission timings of twofeedbacks, the feedbacks are prioritized based on interference typesinvolved in the feedbacks. However, if the two feedbacks have the sameinterference type, feedback information with a lower CSI-RS resourceindex is transmitted with priority.

If the former method is adopted in a DB system, a BS first receives afeedback for a situation in which a UE receives data from a specificcell in various interference situations. However, if the latter methodis adopted in a DS system, a BS first receives a feedback for asituation where a UE receives data from different cells in the sameinterference situation.

In the methods of prioritizing feedbacks in the case of a collisionbetween the transmission timings of the feedbacks, RI transmission haspriority over wCQI or wCQI/PMI transmission, or wCQI or wCQI/PMItransmission has priority over sCQI transmission, at a transmissiontiming at which feedbacks with the same feedback priority levelscollide.

However, the above-described feedback prioritizing methods are used onlyat a transmission timing at which the same type of feedback informationcollides, in such a manner that a higher priority level is given to RItransmission than wCQI or wCQI/PMI transmission and a higher prioritylevel is given to wCQI or wCQI/PMI transmission than sCQI transmission.

Another method for transmitting CSI feedbacks regarding a measurementset assigned to a UE is to divide the measurement set into one or moresubsets and setting a CSI feedback mode and timing for each subset, forCSI feedback. In a method for transmitting a CSI feedback on a subsetbasis, a UE-Preferred cell Index (PI) is included on a subset basis andonly a feedback for a cell corresponding to the PI is transmitted.PI-including feedback methods according to Embodiments 2, 3 and 4 willbe described below in conjunction with DS, DS/DB, and JT.

Embodiment 2

Another embodiment of the present invention provides a method fortransmitting feedbacks in allocated feedback modes at allocated timingsat a CoMP UE in a cellular mobile communication system using DS.

In this embodiment, a BS indicates a measurement set and a plurality ofsubsets of the measurement set to a UE and allocates a feedback mode andtiming for each subset to the UE. For instance, assuming that themeasurement set is {Cell-1, Cell-2, Cell-3, Cell-4} and the BS wants toreceive CSI for UE-preferred two cells among the cells of the allocatedmeasurement set, the BS allocates two feedback modes and timings to theUE and indicates a subset corresponding to the respective feedback modesand timings to the UE. In the example, the following two feedback modesand timings and the subsets corresponding to the feedback modes andtimings are set:

Example 2

-   -   Feedback allocation 1: (Mode A, N_(pd)=10, M_(RI)=2, M_(PI)=2,        N_(OFFSET,CQI)=0, N_(OFFSET,RI)=−1, N_(OFFSET,PI)=−1, {Cell-1,        Cell-2, Cell-3, Cell-4})    -   Feedback allocation 2: (Mode A, N_(pd)=10, M_(RI)=2, M_(PI)=2,        N_(OFFSET,CQI)=3, N_(OFFSET,RI)=−1, N_(OFFSET,PI)=−1, {Cell-1,        Cell-2, Cell-3, Cell-4})

In Example 2, Mode A indicates that feedback information transmittedfrom a UE includes a PI in addition to conventionally included RI, PMI,and CQI information. M_(PI) and N_(OFFSET,PI) are parametersrepresenting the feedback period and offset of the PI, respectively,which can be defined as

-   -   PI period=RI_period×M_(PI)    -   PI offset=RI_offset+N_(OFFSET,PI)        where N_(OFFSET,PI) is an integer ranging from 0 to −N_(pd)+1.        Alternatively or additionally, the PI is encoded jointly with        the RI and transmitted to the BS without the need for separately        defining the period and offset of the PI.

Another method for defining the feedback timing of a PI is to set theperiod of the PI to a multiple of a wCQI period independently of thetiming of an RI and apply an additional offset. That is, the period andoffset of the PI are defined as

-   -   PI period=wCQI_period×M_(PI)    -   PI offset=wCQI_offset+N_(OFFSET,PI)

In the above case, the period and offset of the PI are set toN_(pd)·_(PI) and N_(OFFSET,CQI)+N_(OFFSET,PI), respectively in Example2. When a PI is added to the conventional Mode 2-0 in which an sCQI isdefined, the period and offset of the PI are set to N_(pd)·H·M_(PI) andN_(OFFSET,CQI)+N_(OFFSET,PI), respectively. In the case in which theperiod of a PI is set to a multiple of the period of a wCQI and insteadof an RI, a CQI/PMI is transmitted shortly after the PI, a CQI/PMIrelated to the latest RI and PI or a CQI/PMI set assuming a fixed RI isfed back. The fixed RI is for rank 1, which offers the benefit ofensuring a normal operation when an available RI is different for eachcell. That is, if a first cell supports up to rank 4 and a second cellmerely supports rank 2, an RI feedback for the first cell is not appliedto the second cell. The smaller value between a rank for the latest RIfeedback and a maximum rank available in a cell related to a new PI isanother assumption for an RI in generating a CQI/PMI, when the CQI/PMI,not the RI, is transmitted shortly after a PI. The maximum availablerank in the cell related to the new PI is based on a CSI-RS antenna portsetting or is a maximum rank freely set by the BS. The following threeassumptions can be summarized for an RI in generating a CQI/PMI in thesituation in which the CQI/PMI is transmitted, instead of the RI,shortly after a PI:

-   -   the latest feedback RI value;    -   a fixed RI value (the rank is set to 1); and    -   the minimum between the latest feedback RI value and a maximum        rank for a new PI.

In feedback allocation 2 of Example 2, the constraint that a PI isselected after a PI selected in feedback allocation 1 is excluded froman allocated subset is imposed. A UE feeds back different informationaccording to the number of CSI-RS antenna ports in the cell indicated bya PI. For example, if Cell-1 has 8 antenna ports, each of the othercells has 4 or fewer antenna ports, and a PI indicates Cell-1, feedbackinformation includes two types of PMIs or PTIs in addition to an RI anda CQI corresponding to the 8 antennas. However, if the PI indicatesCell-2, the feedback information has to only include an RI, a CQI, andone type of PMI. The PI occupies 1 or more bits. Table 4 illustrates anexample of the indexes of cells indicated by a 2-bit PI.

TABLE 4 PI field Cell index 00 1 10 2 11 3 01 4

In another example, if a measurement set is {Cell-1, Cell-2, Cell-3,Cell-4} and the BS wants to get information about one preferred cellbetween Cell-1 and Cell-2 and information about one preferred cellbetween Cell-3 and Cell-4, the BS allocates feedbacks as follows:

Example 3

-   -   feedback allocation 1: (Mode A, N_(pd)=10, M_(RI)=2, M_(PI)=2,        N_(OFFSET,CQI)=0, N_(OFFSET,RI)=−1, N_(OFFSET,PI)=−1, {Cell-1,        Cell-2})    -   feedback allocation 2: (Mode A, N_(pd)=10, M_(RI)=2, M_(PI)=2,        N_(OFFSET,CQI)=3, N_(OFFSET,RI)=−1, N_(OFFSET,PI)=−1, {Cell-3,        Cell-4})

As in Example 2, a PI has an independent feedback timing and is jointlyencoded with an RI for transmission in Example 3. A set of cellscorresponding to each feedback is transmitted together with feedbackallocations to a UE, independently of a measurement set or istransmitted to the UE as bitmap information of the measurement set. If abitmap is used for Example 3, the BS transmits a bitmap sequence [1, 1,0, 0] regarding the measurement set in feedback allocation 1 and abitmap sequence [0, 0, 1, 1] regarding the measurement set in feedbackallocation 2. FIG. 9 illustrates feedback transmission timings andfeedback information of a UE regarding two feedback allocations inExample 3 and Example 4. Example 3 and Example 4 are extensions of theconventional Mode 1-1 for 4 or fewer CSI-RS antenna ports. When Mode 2-1is extended, the BS additionally transmits K to the UE. Similarly, theperiod of a PI is also set to an M_(PI) multiple of an RI period and theoffset of the PI is set to the sum of an RI offset and N_(OFFSET,PI). Orthe PI is jointly encoded with the RI without additionally defining a PItiming. Additionally, the period and offset of the PI is set to anM_(PI) multiple of a WCI period and the sum of a wCQI offset andN_(OFFSET,PI), respectively. In a feedback mode for 8 CSI-RS antennaports, including two types of PMIs and two types of PTIs additionally, aUE feeds back a PI additionally in the conventional feedback structureas in Example 2 and Example 3.

In another example, if the measurement set is {Cell-1, Cell-2, Cell-3,Cell-4} and the BS always wants to get channel information about Cell-1and information about a UE-preferred cell from among Cell-2, Cell-3 andCell-4, the BS allocates feedbacks as follows:

Example 4

-   -   feedback allocation 1: (Mode 1-1, N_(pd)=10, M_(RI)=2,        N_(OFFSET,CQI)=0, N_(OFFSET,RI)=−1, {Cell-1})    -   feedback allocation 2: (Mode A, N_(pd)=10, M_(RI)2,        N_(OFFSET,CQI)−3, N_(OFFSET,RI)−1, {Cell-2, Cell-3, Cell-4})

In Example 4, since a cell set includes a single element in feedbackallocation 1, there is no need to define a PI period. When a cell setincludes only one element in a feedback allocation, a PI period is notdefined, or even though a PI period is defined, a PI is not fed back.

In accordance with an embodiment of the present invention, since a UEfeeds back channel information about a plurality of cells at differenttransmission timings, a collision occurs between the transmissiontimings of the channel information. A feedback for a feedback allocationhaving a higher priority is always transmitted first. That is, when oneof an RI, a PMI, and a CQI collides with another, information for ahigher-priority feedback allocation is first fed back. This methodenables a UE to receive data from at least one cell without performancedegradation caused by multi-feedback collision, despite collisionbetween cells. Cells are prioritized by signaling priority informationtogether with feedback allocations to a UE by a BS and determiningfeedback priority levels according to the priority information by theUE. Or, the priority levels of cells are determined based on the indexesof feedback allocations. Or, a higher priority level is assigned to afeedback allocation having the shortest feedback period.

If a PI cannot be transmitted due to a timing collision, a subsequentRI, PMI and CQI is fed back based on a preset PI value. The PI value isindicated to the UE by higher-layer signaling or is determined to be afeedback for a cell having the smallest index in a cell set in afeedback allocation. When an RI cannot be transmitted, a PMI and a CQIare calculated on the same assumption as non-PI transmission.Consequently, the feedback prioritization methods according to the firstembodiment of the present invention are performed in a similar manner inthe second embodiment of the present invention.

Embodiment 3

A third embodiment of the present invention provides a method fortransmitting a feedback in an allocated feedback mode at an allocatedtiming at a CoMP UE in a cellular mobile communication system usingDS/DB.

To implement DS/DB, a UE feeds back to a BS both channel information incase of Interference Cell (I-cell) off and channel information in caseof I-cell on. These two feedbacks are performed independently. However,since an RI and PMI representing spatial information between the UE anda specific cell do not change much in both cases, the RI and PMI are setto the same values in both cases and different CQI values are set inboth cases.

For example, after feedback information for the case of I-cell-on is setas a primary feedback or reference feedback, an RI and PMI for the caseof I-cell off are set to the RI and PMI of the primary feedback (i.e.the reference feedback). The UE calculates a CQI for the case of I-celloff based on at least one of the set RI and PMI, thus setting the CQIseparately from a CQI for the case of I-cell on. As the same RI and PMIare set in both I-cell off case and I-cell-on case, channel informationfor the I-cell off case may includes only the CQI.

A UE acquires channel information in the case of I-cell off and in thecase of I-cell on in two methods. One of the channel informationacquisition methods is that a BS notifies a UE of the index of an I-cellalong with a measurement set. For example, if the BS indicates {Cell-1,Cell-2, Cell-3} as the measurement set and Cell-1 as an I-cell, the UEhas only to feed back channel information only for an I-cell off caseregarding Cell-1. Moreover, the UE should feed back channel informationfor Cell-1 off and Cell-1 in cases regarding Cell-2 and Cell-3.Regarding Cell-2 and Cell-3, a common RI and PMI and two different CQIsare fed back for both Cell-1 on and Cell-1 off cases. Let a CQI for theI-cell on case be called a DS-CQI and a CQI for the I-cell off case becalled a DB-CQI. The DS-CQI and the DB-CQI are fed back at the same ordifferent timings. In the former case, the DB-CQI is fed back as aseparate value or a delta_DB-CQI being the difference from the DS-CQI isfed back. If the RI is 1, a CQI for one codeword is fed back. Then, thedelta_DB-CQI is defined simply as (DB-CQI−DS-CQI). However, if the RI is2 or higher, CQIs for two codewords should be fed back. In theconventional LTE-Advanced technology, a CQI for the second codeword isfed back as the difference from the CQI for the first CQI. That is, whenDS-CQIs for the first and second codewords are respectively calledDS-CQI_CW1 and DS-CQI_CW2, the DS-CQI_CW1 and a delta_DS-CQI_CW2 are fedback conventionally. Herein, delta_DS-CQI_CW2=DS-CQI_CW2−DS-CQI_CW1. IfCQIs for two codewords are needed and DB-CQIs for the first and secondcodewords are DB-CQI_CW1 and DB-CQI_CW2, respectively, a commondelta_DB-CQI is transmitted for the two codewords and used as follows:

-   -   DB-CQI_CW1=DS-CQI_CW1+delta_DB-CQI    -   DB-CQI_CW2=DS-CQI_CW2+delta_DB-CQI

For the two codewords, delta_DB-CQI_CW1 and delta_DB-CQI_CW2 are fedback respectively and used as follows.

-   -   DB-CQI_CW1=DS-CQI_CW1+delta_DB-CQI_CW1    -   DB-CQI_CW2=DS-CQI_CW2+delta_DB-CQI_CW2

As in the second embodiment of the present invention, a BS notifies a UEof a measurement set and a plurality of subsets of the measurement setand allocates a feedback mode and timing for each subset to the UE inthe third embodiment of the present invention. However, the thirdembodiment of the present invention differs from the second embodimentof the present invention in that a DB-CQI or delta_DB-CQI is included ineach feedback. For example, if the measurement set is {Cell-1, Cell-2,Cell-3, Cell-4}, an I-cell is Cell-1, and the BS wishes to receivechannel information about two UE-preferred cells selected from themeasurement set and to receive a delta_DB-CQI and a DS-CQI at the sametiming, the BS allocates two feedback modes and timings to the UE andindicates a subset of the measurement set corresponding to the feedbackmodes and timings to the UE. In this example, the two feedback modes andtimings and the subset are set as follows.

Example 5

-   -   feedback allocation 1: (Mode B, N_(pd)=10, M_(RI)=2, M_(RI)=2,        N_(OFFSET,CQI)=0, N_(OFFSET,RI)=−1, N_(OFFSET,PI)=−1, {Cell-1,        Cell-2, Cell-3, Cell-4})    -   feedback allocation 2: (Mode B, N_(pd)=10, M_(RI)=2, M_(PI)=2,        N_(OFFSET,CQI)=3, N_(OFFSET,RI)=−1, N_(OFFSET,PI)=−1, {Cell-1,        Cell-2, Cell-3, Cell-4})

In Example 5, unlike Mode A of Example 2, Mode B indicates that feedbackinformation to be transmitted from the UE includes a delta_DB-CQI inaddition to a conventionally transmitted RI, PMI, and CQI, and a PI. Thedelta_DB-CQI is jointly encoded with a DS-CQI and fed back at the sametiming. That is, the transmission period and offset of the DS-CQI andthe delta_DB-CQI are set to N_(pd) and N_(OFFSET,PI), respectively. Ifthe PI indicates an I-cell, a DB-CQI is not needed. Therefore, thedelta_DB-CQI is set to 0 or not transmitted, while only the DS-CQI istransmitted. FIG. 10 illustrates feedback transmission timings andfeedback information of a UE in two feedback allocations, when a PIindicates an I-cell in feedback allocation 1 and the PI indicates a cellother than the I-cell in feedback allocation 2. Example 5 is anextension of the conventional Mode 1-1. When Mode 2-1 is extended, theBS will additionally transmit K to the UE. Likewise, the period of a PIis set to an M_(PI) multiple of an RI period and the offset of the PI isset to the sum of an RI offset and N_(OFFSET,PI). Or, the PI is jointlyencoded with the RI and transmitted together without defining anadditional PI timing. The period and offset of a PI for a wCQI aredefined, respectively, as an M_(PI) multiple of the period of the wCQIand the sum of the offset of the wCQI and N_(OFFSET,PI). In an extensionof Mode 2-1, a delta_DB-CQI is defined for each of a wCQI and an sCQIand transmitted together with a DS-CQI, or a delta_DB-CQI is defined forone of the wCQI and the sCQI and transmitted together with a DS-CQI at agiven timing. In a feedback mode for 8 CSI-RS antenna ports, whichadditionally includes two types of PMIs and two types of PTIs, the UEadditionally feeds back a PI and a delta_DB-CQI in the conventionalfeedback structure in the same manner as in Example 5.

In another example, if the measurement set is {Cell-1, Cell-2, Cell-3,Cell-4} and an I-cell is Cell-1 and the BS wants to receive channelinformation about two UE-preferred cells selected from the measurementset and to receive a DB-CQI and a DS-CQI at different timings, the BSconfigures two feedback modes, timing information about the transmissiontiming of the DB-CQI, and a subset of the measurement set correspondingto the feedback modes and timing, as follows.

Example 6

-   -   feedback allocation 1: (Mode C, N_(pd)=10, M_(RI)=2, M_(PI)=2,        H″=2, N_(OFFSET,CQI)=−0, N_(OFFSET,RI)=−1, N_(OFFSET,PI)=−1,        {Cell-1, Cell-2, Cell-3, Cell-4})    -   feedback allocation 2: (Mode C, N_(pd)=10, M_(RI)=2, M_(PI)=2,        H″=2, N_(OFFSET,CQI)=3, N_(OFFSET,RI)=−1, N_(OFFSET,PI)=−1,        {Cell-1, Cell-2, Cell-3, Cell-4})

In Example 6, Mode C includes a parameter H″ indicating the period of aDB-CQI, compared to Mode B of Example 5. The transmission period andoffset of the DB-CQI are set to H″·N_(pd) and N_(OFFSET,CQI),respectively. That is, in the situation in which the DS-CQI istransmitted in a period of N_(pd), a DB-CQI is transmitted every H″times. Instead of the DB-CQI, a delta_DB-CQI is transmitted. Moreover,in Example 6, when the PI indicates an I-cell, the DB-CQI is not neededand thus only the DS-CQI is transmitted. FIG. 11 illustrates feedbacktransmission timings and feedback information of a UE in two feedbackallocations, when a PI indicates an I-cell in feedback allocation 1 andthe PI indicates a cell other than the I-cell in feedback allocation 2.Example 6 is an extension of the conventional Mode 1-1. When Mode 2-1 isextended, the BS additionally transmits K to the UE. Likewise, theperiod of a PI is set to an M_(PI) multiple of an RI period and theoffset of the PI is set to the sum of an RI offset and N_(OFFSET,PI).Or, the PI is jointly encoded with the RI and transmitted togetherwithout defining an additional PI timing. The period and offset of a PIfor a wCQI are defined respectively as an M_(PI) multiple of the periodof the wCQI and the sum of the offset of the wCQI and N_(OFFSET,PI). Inan extension of Mode 2-1, a DB-CQI is defined for each of a wCQI and ansCQI and transmitted together with a DS-CQI, or a DB-CQI is defined forone of the wCQI and the sCQI and transmitted together with a DS-CQI at agiven timing. The transmission timing of a DB-sCQI, i.e. a DB-CQI for ansCQI, is set such that (H−1) DB-sCQIs are transmitted at an interval ofN_(pd) subframes after a timing of the transmission period of a DB-wCQI,i.e. a DB-CQI for a wCQI, H″·N_(pd) and the offset N_(OFFSET,CQI).

Example 7 is an example of available feedback allocations, when a DB-CQIis transmitted by extending Mode 2-1. FIG. 12 illustrates feedbacktransmission timings and feedback information of a UE in two feedbackallocations, when a PI indicates an I-cell in feedback allocation 1 andthe PI indicates a cell other than the I-cell in feedback allocation 2.

Example 7

-   -   feedback allocation 1: (Mode D, N_(pd)=5, M_(RI)=2, M_(PI)=2,        J=3 (10 MHz), K=1, H″=2, N_(OFFSET,CQI)−0, N_(OFFSET,RI)=−1,        N_(OFFSET,PI)=1, {Cell-1, Cell-2, Cell-3, Cell-4})    -   feedback allocation 2: (Mode D, N_(pd)=5, M_(RI)=2, M_(PI)=2,        J=3 (10 MHz), K=1, H″=2, N_(OFFSET,CQI)=3, N_(OFFSET,RI)=−1,        N_(OFFSET,PI)=−1, {Cell-1, Cell-2, Cell-3, Cell-4})

In Example 7, Mode D is a new feedback mode that is an extension of theconventional Mode 2-1 requiring additional transmission of a DB-CQI. Ina feedback mode for 8 CSI-RS antenna ports, including two types of PMIsand two types of PTIs additionally, a UE feeds back a PI and a DB-CQIadditionally in the conventional feedback structure in the same manneras in Example 7.

Another method for acquiring CSI, i.e. a DB-CQI and a DS-CQI in bothI-cell on and I-cell off cases at a UE is that a BS indicates an IMR foreach cell as well as a measurement set to the UE. That is, when the BSsignals CSI-RS resources for signal measurement, an IMR for an I-cell oncase, and an IMR for an I-cell off case regarding each cell of themeasurement set to the UE, the UE acquires both a DB-CQI and a DS-CQIfor the cell based on channel information acquired from the resources.For a cell in which only one IMR is known, the UE only has to calculatea DS-CQI.

For example, if the BS allocates {Cell-1, Cell-2, Cell-3} as themeasurement set and indicates to the UE one IMR for Cell-1 and two IMRsfor each of Cell-2 and Cell-3, the UE feeds back one piece of channelinformation for Cell-1 and channel information for two cases for each ofCell-2 and Cell-3. The RI and PMI included in the channel informationabout two IMRs for each of Cell-2 and Cell-3 are set to the same values.Different CQIs are generated for the two cases based on at least one ofthe set RI and PMI. If the smaller between two CQIs is set as a DS-CQIand the larger CQI is set as a DB-CQI, the DS-CQI and the DB-CQI are fedback to the BS in a specific feedback mode at a specific timing. Thefeedback mode and timing are set in the methods described in Example 5,Example 6, and Example 7.

The UE classifies two pieces of channel information generated for eachof Cell-2 and Cell-3 into primary CSI and secondary CSI. For instance,the UE sets channel information including the smaller between two CQIsas primary CSI and the other channel information as secondary CSI. TheRI and PMI included in the secondary CSI are set to the same values asthe RI and PMI included in the primary CSI, and a CQI for the secondaryCSI is calculated to be feedback based on at least one of the set RI andPMI. Since the RI and PMI of the secondary CSI are set based on the RIand PMI of the primary CSI, the primary CSI is used as a reference CSIfor the secondary CSI.

Embodiment 4

A fourth embodiment of the present invention provides a method fortransmitting a feedback in an allocated feedback mode at an allocatedtiming at a CoMP UE in a cellular mobile communication system using JT.

To implement JT, a UE needs to feed back channel information in the caseof simultaneous transmission from some cells of a measurement set aswell as channel information for each cell of the measurement set to aBS. Aside from channel information for each cell, an RI, PMI, wCQI, andsCQI based on cooperation between cells are referred to as a JT_RI,JT_PMI, JT_wCQI, and JT_sCQI, respectively. The UE feeds back all or apart of the cooperation information to the BS. Specifically, the JT_RIis not included in the feedback information because the BS can estimatethe JT_RI based on the channel information for each cell. Additionally,the JT_PMI is configured to include only phase difference informationbetween cells that consider cooperation, rather than being configured tobe a PMI for JT. If the JT_wCQI and the JT_sCQI are generically called aJT_CQI, this value is a CQI needed for a cooperation situation or a CQIdifference between a cooperation case and a non-cooperation case. Whenthe JT_CQI is defined as a CQI difference, it is defined commonly orseparately for two codewords, like a delta_DB-CQI defined in the thirdembodiment of the present invention.

Like a DB-CQI, channel information for coordinated transmission isencoded and transmitted jointly with individual feedbacks for cells of ameasurement set at the same timing, or is fed back in a differentfeedback mode at a different timing from the individual feedbacks forthe cells. The case of transmitting information for coordinatedtransmission together with individual feedbacks for the cells of ameasurement set will be described. Both or either of the JT_CQI andJT_PMI are transmitted while the JT_RI is not transmitted.

In accordance with the fourth embodiment of the present invention, whena JT feedback is transmitted at the same timing as a DS-CQI for anindividual cell, the BS indicates a measurement set and a plurality ofsubsets of the measurement set to the UE and allocates a feedback modeand timing to each subset as in the third embodiment of the presentinvention. The difference between the fourth embodiment and the thirdembodiment of the present invention is that a DB-CQI is replaced with aJT-CQI/JT-PMI in each feedback. For example, if the measurement set is{Cell-1, Cell-2, Cell-3, Cell-4} and the BS needs channel informationabout two UE-preferred cells selected from the measurement set and a JTfeedback based on cooperation between the cells, the BS allocates twofeedback modes and timings to the UE and indicates a subset of themeasurement set corresponding to the feedback modes and timings to theUE. In this example, the two feedback modes and timings and the subsetare set as follows:

Example 8

-   -   feedback allocation 1: (Mode A, N_(pd)=10, M_(RI)=2, M_(PI)=2,        N_(OFFSET,CQI)=0, N_(OFFSET,RI)=−1, N_(OFFSET,PI)=−1, {Cell-1,        Cell-2, Cell-3, Cell-4})    -   feedback allocation 2: (Mode E, N_(pd)=10, M_(RI)=2, M_(PI)=2,        N_(OFFSET,CQI)=3, N_(OFFSET,RI)==1, N_(OFFSET,PI)=−1, {Cell-1,        Cell-2, Cell-3, Cell-4}, feedback allocation 1)

In Example 8, Mode A is identical to Mode A defined in Example 2,meaning that the UE needs to feedback a conventionally transmitted RI,PMI and CQI and a PI. Mode E is a mode indicating additionaltransmission of a JT feedback in Mode A. In Mode E, the UE feeds back aJT feedback based on cooperation between a cell corresponding to a PIselected in the indicated feedback allocation 1 and a cell to beselected in feedback allocation 2 to the BS at a CQI transmissiontiming. Accordingly, when Mode E is indicated as the feedback mode, theindex of a feedback allocation for which cooperation is indicated to theUE and the feedback allocation index is directly signaled to the UE bythe BS through higher-layer signaling or is preset to feedbackallocation 1. The JT_CQI/JT-PMI is then fed back with a DS-CQI in atransmission period of N_(pd) with an offset of N_(OFFSET,CQI).

FIG. 13 illustrates feedback transmission timings and feedbackinformation of a UE in two feedback allocations regarding Example 8.Example 8 is an extension of the conventional Mode 1-1. When Mode 2-1 isextended, the BS additionally transmits K to the UE. The period of a PIis set to an M_(PI) multiple of an RI period and the offset of the PI isset to the sum of an RI offset and N_(OFFSET,PI). Or, the PI is jointlyencoded with the RI and transmitted together without defining anadditional PI timing. The period and offset of a PI for a wCQI aredefined respectively as an M_(PI) multiple of the period of the wCQI andthe sum of the offset of the wCQI and N_(OFFSET,PI), respectively. In anextension of Mode 2-1, both a JT_wCQI and a JT_sCQI are defined andtransmitted together with a DS-CQI, or one of the JT_wCQI and theJT_sCQI is defined and transmitted together with a DS-CQI at a giventiming. In a feedback mode for 8 CSI-RS antenna ports, whichadditionally includes two types of PMIs and two types of PTIs, the UEadditionally feeds back a PI and a JT_CQI/JT_PMI in the conventionalfeedback structure in the same manner as in Example 8.

In another example, if the measurement set is {Cell-1, Cell-2, Cell-3,Cell-4} and the BS wants to receive channel information about twoUE-preferred cells selected from the measurement set, to receive a JTfeedback based on cooperation between the cells, and to receive aJT_CQI/JT_PMI at a different timing from a DS-CQI, the BS configures twofeedback modes, timing information about the transmission timing of theJT_CQI/JT_PMI, and a subset of the measurement set corresponding to thefeedback modes and timing, as follows.

Example 9

-   -   feedback allocation 1: (Mode A, N_(pd)=10, M_(RI)=2, M_(PI)=2,        N_(OFFSET,CQI)=0, N_(OFFSET,RI)=−1, N_(OFFSET,PI)=−1, {Cell-1,        Cell-2, Cell-3, Cell-4})    -   feedback allocation 2: (Mode F, N_(pd)=10, M_(RI)=2, M_(PI)=2,        H″=2, N_(OFFSET,CQI)=3, N_(OFFSET,RI)−1, N_(OFFSET,PI)=−1,        {Cell-1, Cell-2, Cell-3, Cell-4}, feedback allocation 1)

In Example 9, Mode F includes a parameter H″ indicating the period of aJT_CQI/JT_PMI, compared to Mode E in Example 8. The transmission periodand offset of the JT_CQI/JT_PMI are set to H″·N_(pd) and N_(OFFSET,CQI),respectively. That is, in a situation in which a DS-CQI is transmittedin a period of N_(pd), a JT_CQI/JT_PMI is transmitted every H″ times.FIG. 14 illustrates feedback transmission timings and feedbackinformation of a UE in two feedback allocations, with respect to Example9. Example 9 is an extension of the conventional Mode 1-1. When Mode 2-1is extended, the BS additionally transmits K to the UE. Likewise, theperiod of a PI is set to an M_(PI) multiple of an RI period and theoffset of the PI is set to the sum of an RI offset and N_(OFFSET,PI).Or, the PI is jointly encoded with the RI and transmitted togetherwithout defining an additional PI timing. The period and offset of a PIfor a wCQI are defined respectively as an M_(PI) multiple of the periodof the wCQI and the sum of the offset of the wCQI and N_(OFFSET,PI). Inan extension of Mode 2-1, both a JT_wCQI and a JT_sCQI are defined andtransmitted together with a DS-CQI, or only one of the JT_wCQI and theJT_sCQI is defined and transmitted together with a DS-CQI at a giventiming. The transmission timing of the JT_sCQI is set such that (H−1)DB-sCQIs are transmitted at an interval of N_(pd) subframes after atiming of the transmission period of a JT_wCQI, H″·N_(pd) and its offsetN_(OFFSET,CQI). In a feedback mode for 8 CSI-RS antenna ports, whichadditionally includes two types of PMIs and two types of PTIs, the UEadditionally feeds back a JT_CQI/JT_PMI in the conventional feedbackstructure in the same manner as in Example 9.

Channel information for coordinated transmission is fed back in adifferent mode at a different timing from an individual feedback foreach cell. For instance, if the measurement set is {Cell-1, Cell-2,Cell-3, Cell-4} and the BS wants to receive channel information abouttwo UE-preferred cells selected from the measurement set, to receive aJT feedback based on cooperation between the cells, and to receive theJT feedback separately from channel information for each cell, the BSconfigures two feedback modes for individual cell feedbacks, and onefeedback mode for JT, as follows.

Example 10

-   -   feedback allocation 1: (Mode A, N_(pd)=10, M_(RI)=2, M_(PI)=2,        N_(OFFSET,CQI)=0, N_(OFFSET,RI)=−1, N_(OFFSET,PI)=−1, {Cell-1,        Cell-2, Cell-3, Cell-4})    -   feedback allocation 2: (Mode A, N_(pd)=10, M_(RI)=2, M_(PI)=2,        N_(OFFSET,CQI)=3, N_(OFFSET,RI)=−1, N_(OFFSET,PI)=−1, {Cell-1,        Cell-2, Cell-3, Cell-4})    -   feedback allocation 3: (Mode G, N_(pd)=10, M_(RI)=2,        N_(OFFSET,CQI)=6, N_(OFFSET,RI)=−1, {feedback allocation 1,        feedback allocation 2})

In Example 10, Mode G is a mode in which a JT_RI, JT_PMI and JT_CQI aretransmitted as feedback information based on JT between cells selectedfrom feedback allocation 1 and feedback allocation 2. For the JTfeedback, feedback periods and offsets are set as in the conventionalMode 1-1 and a feedback allocation number for JT is also set. Thefeedback allocation number is signaled separately to the UE. Or thefeedback allocation number is for cooperation between predeterminedfeedback allocation numbers or between predetermined cells. A JT_RI isnot fed back and only a JT_PMI and a JT_CQI are transmitted. Thus,M_(RI) and N_(OFFSET,RI) are not set.

FIG. 15 is a block diagram illustrating a UE according to an embodimentof the present invention.

Referring to FIG. 15, the UE includes a communication module 710 and acontroller 720.

The communication module 710 transmits or receives data externally. Thecommunication module 710 transmits channel information for CoMP to acentral control device under the control of the controller 720.

The controller 720 controls the states and operations of all componentsin the UE. The controller 720 selects feedback information for the bestcell or coordinated communication according to the communication statesbetween the UE and cells and feed back channel information about theselected cell to the central control device. For this purpose, thecontroller 720 includes a channel estimator 730.

The channel estimator 730 determines feedback information on a CSI-RSbasis according to CoMP set information and feedback mode informationreceived from the central control device and estimates channels usingreceived CSI-RSs. The channel estimator 730 feeds back CoMP-relatedchannel information to the central control device by controlling thecommunication module 710.

While the UE is shown as including the communication module 710 and thecontroller 720, the UE is not limited to the specific configuration.That is, the UE further includes many other components according to itsfunctions. For example, the UE additionally includes a display fordisplaying the current state of the UE, an input unit for receiving asignal such as a function execution command from a user, a memory forstoring data generated from the UE, etc.

FIG. 16 is a flowchart illustrating an operation of the UE according toan embodiment of the present invention.

Referring to FIG. 16, the UE receives CoMP set information and feedbackmode information from the central control device in step 801 anddetermines necessary feedbacks on a CSI-RS basis based on the receivedinformation in step 802. In step 803, the UE estimates channels usingCSI-RSs based on the determined feedbacks. The UE then feeds backCoMP-related channel information to the central control device accordingto the channel estimation result in step 804.

FIG. 17 is a block diagram of a central control device according to anembodiment of the present invention.

The central control device includes a controller 910 and a communicationmodule 920.

The controller 910 controls the states and operations of all componentsin the central control device. The controller 910 allocates CSI-RSs toresources on a cell basis, for channel estimation at the UE, allocates aCoMP set when needed, and transmits CoMP set information to the UE. Forthis purpose, the controller 910 further includes a per-cell resourceallocator 930.

The per-cell resource allocator 930 allocates resources to CSI-RSs forchannel estimation at the UE on a cell basis and transmits CSI-RSs inthe allocated resources through the communication module 920. Resourcesfor each cell are allocated in correspondence with CSI-RSs for channelestimation.

The communication module 920 transmits and receives data to and from theUE or a managed cell. The communication module 920 transmits CSI-RSs tothe UE in the allocated resources and receives feedback informationunder the control of the controller 910.

FIG. 18 is a flowchart illustrating an operation of the central controldevice according to an embodiment of the present invention.

Referring to FIG. 18, the central control device allocates resources toCSI-RSs for channel estimation of a UE on a cell basis in step 1001, andtransmits CSI-RSs in the resources to the UE in step 1002. In step 1003,the central control device receives feedback information includingchannel information from the UE.

As is apparent from the above description of the present invention, inthe case in which DB and JT are used, an effective feedback can beperformed by setting the types, periods, and timings of feedbackinformation, taking into account multi-CSI feedbacks in an LTE-A CoMPsystem where a plurality of BSs supports coordinated downlinktransmission to a UE.

While the present invention has been shown and described with referenceto certain embodiments and drawings of the portable terminal, it will beunderstood by those skilled in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the invention as defined by the appended claims and theirequivalents.

What is claimed is:
 1. A method for transmitting feedback information bya User Equipment (UE) in a mobile communication system, the methodcomprising: receiving information about at least one Channel StateInformation Reference Signal (CSI-RS); performing channel estimationbased on the information about at least one CSI-RS; receivinginformation about at least one feedback; and transmitting at least onefeedback including a channel estimation result using the informationabout at least one feedback, wherein the information about at least onefeedback includes information about a transmission timing and a feedbackmode for the at least one feedback.
 2. The method of claim 1, whereinthe information about at least one feedback further includes informationabout at least one Interference Measurement Resource (IMR), and whereinthe information about at least one feedback further includes at leastone feedback IDentifier (ID) one-to-one mapped to the information aboutat least one IMR and the information about at least one CSI-RS.
 3. Themethod of claim 2, further comprising: if information about a pluralityof feedbacks including information about a same timing are present,transmitting information about one of the plurality of feedbacks at acorresponding timing according to priority levels of the plurality offeedbacks determined based on feedback IDs of the plurality offeedbacks.
 4. The method of claim 2, further comprising: if a pluralityof feedbacks including information about a same timing are present,transmitting information about one of the plurality of feedbacks at acorresponding timing according to priority levels of the plurality offeedbacks determined based on information about at least one of the IMRinformation and the CSI-RS information corresponding to the plurality offeedbacks.
 5. The method of claim 1, wherein the at least one feedbackincludes a first feedback including a primary CSI and a second feedbackincluding a secondary CSI, the secondary CSI is calculated by using atleast one of a Rank Indicator (RI) and a Precoding Matrix Index (PMI) tothe same as value as a RI and a PMI for the primary CSI.
 6. A method forreceiving feedback information by a central control device managing atleast one cell in a mobile communication system, the method comprising:transmitting information about at least one Channel State InformationReference Signal (CSI-RS) to a User Equipment (UE); transmittinginformation about at least one feedback to the UE; and receiving atleast one feedback transmitted based on the information about at leastone feedback from the UE, wherein the information about at least onefeedback includes a channel estimation result generated based on theinformation about at least one CSI-RS and includes information about atransmission timing and a feedback mode for the at least one feedback.7. The method of claim 6, wherein the information about at least onefeedback further includes information about at least one InterferenceMeasurement Resource (IMR), and wherein the information about at leastone feedback further includes at least one feedback IDentifier (ID)one-to-one mapped to the information about at least one IMR and theinformation about at least one CSI-RS.
 8. The method of claim 7, furthercomprising: if information about a plurality of feedbacks includinginformation about a same timing at the UE are present, receivinginformation about one of the plurality of feedbacks at a correspondingtiming according to priority levels of the plurality of feedbacksdetermined based on feedback IDs of the plurality of feedbacks.
 9. Themethod of claim 7, further comprising: if a plurality of feedbacksincluding information about a same timing at the UE are present,receiving information about one of the plurality of feedbacks at acorresponding timing according to priority levels of the plurality offeedbacks determined based on information about the IMR information andthe CSI-RS information corresponding to the plurality of feedbacks. 10.The method of claim 6, wherein the at least one feedback includes afirst feedback including a primary CSI and a second feedback including asecondary CSI, the secondary CSI is calculated by using at least one ofa Rank Indicator (RI) and a Precoding Matrix Index (PMI) to the same asvalue as a RI and a PMI for the primary CSI.
 11. A User Equipment (UE)in a mobile communication system, the UE comprising: a transmitter; areceiver for receiving information about at least one Channel StateInformation Reference Signal (CSI-RS) and information about at least onefeedback; and a controller for performing channel estimation based onthe information about at least one CSI-RS and, upon receipt of theinformation about at least one feedback, controlling the transmitter totransmit at least one feedback including a channel estimation resultusing the information about at least one feedback, wherein theinformation about at least one feedback includes information about atransmission timing and a feedback mode for the at least one feedback.12. The UE of claim 11, wherein the information about at least one onefeedback further includes information about at least one InterferenceMeasurement Resource (IMR), and wherein the information about at leastone feedback further includes at least one feedback IDentifier (ID)one-to-one mapped to the information about at least one IMR and theinformation about at least one CSI-RS.
 13. The UE of claim 12, wherein,if information about a plurality of feedbacks including informationabout a same timing are present, the controller controls the transmitterto transmit information about one of the plurality of feedbacks at acorresponding timing according to priority levels of the plurality offeedbacks determined based on feedback IDs of the plurality offeedbacks.
 14. The UE of claim 12, wherein, if a plurality of feedbacksincluding information about a same timing are present, the controllercontrols the transmitter to transmit information about one of theplurality of feedbacks at a corresponding timing according to prioritylevels of the plurality of feedbacks determined based on informationabout at least one of the IMR information and the CSI-RS informationcorresponding to the plurality of feedbacks.
 15. The UE of claim 11,wherein the at least one feedback includes a first feedback including aprimary CSI and a second feedback including a secondary CSI, thesecondary CSI is calculated by using at least one of a Rank Indicator(RI) and a Precoding Matrix Index (PMI) to the same as value as a RI anda PMI for the primary CSI.
 16. A central control device managing atleast one cell in a mobile communication system, the central controldevice comprising: a transmitter for transmitting information about atleast one Channel State Information Reference Signal (CSI-RS) andinformation about at least one feedback to a User Equipment (UE); and areceiver for receiving at least one feedback transmitted based on theinformation about at least one feedback from the UE, wherein theinformation about at least one feedback includes a channel estimationresult generated based on the information about at least one CSI-RS andincludes a transmission timing and a feedback mode for the at least onefeedback.
 17. The central control device of claim 16, wherein theinformation about at least one feedback further includes informationabout at least one Interference Measurement Resource (IMR), and whereinthe information about at least one feedback further includes at leastone feedback IDentifier (ID) one-to-one mapped to the information aboutat least one IMR and the information about at least one CSI-RS.
 18. Thecentral control device of claim 17, wherein, if information about aplurality of feedbacks including information about a same timing at theUE, the receiver receives information about one of the plurality offeedbacks at a timing according to priority levels of the plurality offeedbacks determined based on feedback IDs of the plurality offeedbacks.
 19. The central control device of claim 17, wherein, ifinformation about a plurality of feedbacks including information about asame timing at the UE, the receiver receives one of the plurality offeedbacks at a corresponding timing according to priority levels of theplurality of feedbacks determined information about based on the IMRinformation and the CSI-RS information corresponding to the plurality offeedbacks.
 20. The central control device of claim 16, wherein the atleast one feedback includes a first feedback including a primary CSI anda second feedback including a secondary CSI, the secondary CSI iscalculated by using at least one of a Rank Indicator (RI) and aPrecoding Matrix Index (PMI) to the same as value as a RI and a PMI forthe primary CSI.